1. Field of the Invention
The present invention relates to an optical fiber and, more particularly, to an optical fiber arranged in the insertion section of an endoscope or the light-transmitting portion of a camera to transmit illumination light to an object under test.
2. Description of the Related Art
A light guide fiber bundle is generally inserted inside the insertion section of an endoscope to transmit illumination light to a distal end constituent portion of the insertion section. The light guide fiber bundle is guided to a connector through a universal cord connected to the operation section of the endoscope, and an incident end is formed in the connector to receive illumination light.
The connector is connected to a light source unit. The illumination light emitted from a light source is transmitted from the incident end to the light guide fiber bundle and emerges from an exit end serving as an illumination window formed in the distal end constituent portion of the insertion section.
The light guide fiber bundle is constituted by bundling a plurality of light guide fiber elements each having a two-layered structure consisting of a core layer and a cladding layer. A method of coating MoS.sub.2 (molybdenum sulfide) as a solid lubricant is known to reduce friction between the fiber elements and prevent the fiber elements from being broken.
The illumination light is totally reflected by an interface between the core layer and the cladding layer and is then transmitted to the distal end constituent portion of the insertion section of the endoscope through the core layer. Due to micro disorders at the interface between the core layer and the cladding layer, the illumination light partially passes through the interface and enters the cladding layer. The illumination light passing into the cladding layer is incident on and absorbed by MoS.sub.2 coated on the surface of each light guide fiber element. That is, the illumination light incident on the light guide fiber bundle is partially absorbed and attenuated by MoS.sub.2, and the attenuated light is optically transmitted to the distal end constituent portion of the insertion section.
As described above, if MoS.sub.2 is coated on the surface of each light guide fiber element to prevent friction between the fiber elements and damage to the fiber elements, light passing into the cladding layer (i.e., cladding mode light) is discarded outside the fiber element, i.e., valuable illumination light is partially wasted. To obtain a sufficient amount of illumination light, therefore, the number of fiber elements must be increased. As a result, the diameter of the insertion section undesirably increases.
when the glass fiber bundle coated with MoS.sub.2 is subjected to a continuous friction test upon a humidity resistance test, MoS.sub.2 layers stick to each other, and the glass fiber bundle tends to break. Even in a glass fiber bundle not coated with MoS.sub.2, a humidity resistance test causes burning, and the glass fiber bundle tends to break. The glass fiber bundle coated with MoS.sub.2 tends to be contaminated because the MoS.sub.2 layer consists of black grains.